When a function value's mode is BLKmode (and in some other
cases), the value is not returned according to FUNCTION_VALUE
(see section How Scalar Function Values Are Returned). Instead, the caller passes the address of a
block of memory in which the value should be stored. This address
is called the structure value address.

This section describes how to control returning structure values in
memory.

RETURN_IN_MEMORY (type)

A C expression which can inhibit the returning of certain function
values in registers, based on the type of value. A nonzero value says
to return the function value in memory, just as large structures are
always returned. Here type will be a C expression of type
tree, representing the data type of the value.
Note that values of mode BLKmode must be explicitly handled
by this macro. Also, the option `-fpcc-struct-return'
takes effect regardless of this macro. On most systems, it is
possible to leave the macro undefined; this causes a default
definition to be used, whose value is the constant 1 for BLKmode
values, and 0 otherwise.
Do not use this macro to indicate that structures and unions should always
be returned in memory. You should instead use DEFAULT_PCC_STRUCT_RETURN
to indicate this.

DEFAULT_PCC_STRUCT_RETURN

Define this macro to be 1 if all structure and union return values must be
in memory. Since this results in slower code, this should be defined
only if needed for compatibility with other compilers or with an ABI.
If you define this macro to be 0, then the conventions used for structure
and union return values are decided by the RETURN_IN_MEMORY macro.
If not defined, this defaults to the value 1.

STRUCT_VALUE_REGNUM

If the structure value address is passed in a register, then
STRUCT_VALUE_REGNUM should be the number of that register.

STRUCT_VALUE

If the structure value address is not passed in a register, define
STRUCT_VALUE as an expression returning an RTX for the place
where the address is passed. If it returns 0, the address is passed as
an "invisible" first argument.

STRUCT_VALUE_INCOMING_REGNUM

On some architectures the place where the structure value address
is found by the called function is not the same place that the
caller put it. This can be due to register windows, or it could
be because the function prologue moves it to a different place.
If the incoming location of the structure value address is in a
register, define this macro as the register number.

STRUCT_VALUE_INCOMING

If the incoming location is not a register, then you should define
STRUCT_VALUE_INCOMING as an expression for an RTX for where the
called function should find the value. If it should find the value on
the stack, define this to create a mem which refers to the frame
pointer. A definition of 0 means that the address is passed as an
"invisible" first argument.

PCC_STATIC_STRUCT_RETURN

Define this macro if the usual system convention on the target machine
for returning structures and unions is for the called function to return
the address of a static variable containing the value.
Do not define this if the usual system convention is for the caller to
pass an address to the subroutine.
This macro has effect in `-fpcc-struct-return' mode, but it does
nothing when you use `-freg-struct-return' mode.